Blackberry,
The log cabin is one of my favorite classes to teach and I make many of them myself; the block is incredibly versatile.
I square up LC blocks after every "round"....that is, after 4 sides of the center block have been sewn. (I call it four logs being added to make the house square.)
I use a square up ruler, and with the center square (also known as the hearth block) exactly in the middle, I trim tiny amounts off EACH side of the cabin to make it square.
Make note of the measurement the first time you do this, and use it on every LC block at this point. (You may not make all the blocks in one sitting, but you will be able to trim later ones to the same size if you make a note.)
Then I trim again after the next complete "round" of 4 logs added. And again if there is a third "round".
When my blocks have all been made and trimmed, the size they are after the final round is the size they will be for the quilt. To me it doesn't matter if they are the exact size the pattern called for as long as they are all the same size as each other. :idea: Everyone's seam allowance will be slightly different any way; yours is your own Personal Private Measurement. The blocks will be close enough to the size called for in the pattern that it will make a quilt pretty close to the finished size the pattern stated. :D
**Disclaimer: This method of letting the size fall where it may will likely only work for you if you have a quilt with JUST LC blocks in it. If there are other kinds of pieced blocks among the LC blocks, then you'll have to worry about making them all exactly the same size in order to fit together.**
Jan in VA